In 2000, the National Heart, Lung, and Blood Institute (NHLBI) awarded contracts to conduct a randomized, double-blind, placebo-controlled trial in young children with sickle cell disease (SCD) to test the hypothesis that Hydroxyurea (HU) can prevent the onset of chronic end organ damage in children recruited before two years of age. Contracts were awarded to ten Clinical Centers to recruit, enroll, and follow patients to monitor clinical responsiveness to study treatments, to assess growth and development, and to monitor for toxicity from study treatments. Surrogate markers of end organ damage were used to evaluate pulmonary, renal, splenic, and brain function as well as developmental milestones. One Medical Coordinating Center was awarded to oversee drug distribution, coordinate central laboratory functions, perform data collection and analysis, and conduct clinical sites visits to monitor study performance. The trial enrolled 193 subjects with SCD between the ages of 9 and 17 months from October 2003 to June 2007. Subjects remained on study drug for a period of two years. HU demonstrated substantial clinical benefit without serious toxicity in this cohort of very young children. In 2008, a follow-up study was added to the contracts to provide structured follow-up of the children after they completed their two years on study drug. The purpose of the initial Follow-Up Study is to characterize the long-term toxicities and unexpected risks (if any) associated with treatment with HU at an early age. Information obtained from this follow-up study is vitally important to understanding the risks and benefits of early treatment, and ultimately for creation of an optimal paradigm for HU therapy in young children with sickle cell anemia. The follow-up study ends in December 2011. The purpose of the Baby Hug Follow-Up Study II is to provide continued structured follow-up of the children enrolled in the Baby Hug Follow-Up Study I, to characterize the long-term toxicities and unexpected risks (if any) associated with HU treatment at an early age, and to determine if there are clinical benefits from the treatments. Collection and ongoing evaluation of growth and development and clinical data are crucial for determination of the long-term effects of HU. The objective is to intensively monitor and assess this unique group of children for growth, development, and clinical status at least through the first decade of life to document any alterations in the natural history of sickle cell disease associated with early HU therapy. The follow-up will include enhanced neuropsychological, brain, cardiac, and pulmonary evaluations. All children enrolled will be followed to a common termination date of December 31, 2016. Results from the Follow-up Study II will improve understanding of the natural history of SCD in young children and in a cohort receiving HU. Whether or not HU reduces organ damage in these children will be established. If HU has a beneficial effect, the standard of care for children with SCD will be permanently altered.